Sweep generator for linear cathode ray tube display

ABSTRACT

There is disclosed a relatively simple &#39;&#39;&#39;&#39;ramp&#39;&#39;&#39;&#39; generator for a CRT display which generates the necessary correction current to linearize the sweep across a quasi-flat screen. As in prior art systems, an integrator develops a ramp voltage which is applied to the input of a voltage-to-current converter. However, instead of the input to the integrator being a constant current as in the prior art, the input current is constant only when the yoke current is below a predetermined positive or negative limit. Above the limit, the current at the input of the integrator is inversely proportional to the yoke current. Once the feedback becomes effective in this manner, the yoke current becomes an exponential function of time. By providing double-ended feedback, the yoke current is a symmetrical function of time with respect to its positive and negative portions, and the yoke current exhibits no discontinuities in slope.

United States Patent [1 1 Cavigelli 1 SWEEP GENERATOR FOR LINEAR CATI-IODE RAY TUBE DISPLAY [75] Inventor: George A. Cavigelli, Lexington,

Mass.

[73] Assignee: American Optical Corporation,

Southbridge, Mass.

[22] Filed: Aug. 6, 1973 [21] Appl. No.: 386,237

[52] U.S. CI 328/184, 307/228, 315/27 GD [51] Int. Cl. H03k 4/10, HOlj 29/70 [58] Field of Search 307/228; 328/181-185; 315/27 GD [56] References Cited UNITED STATES PATENTS 2,975,369 3/1961 Vance 315/27 GD 3,309,560 3/1967 Popodi 315/27 GD 3,341,654 9/1967 Pay et al.... 307/228 3,364,366 l/1968 Dryden 307/228 3,435,278 3/1969 Carlock et al. 315/27 GD 3,678,332 7/1972 Boekhorst 328/181 3,706,905 12/1972 Alexander 315/27 GD 3,723,805 3/1973 Scarpino et al 315/27 GD OTHER PUBLICATIONS Pincushion Correction by Feedback," by Johnson in [4 1 Feb. 18,1975

IBM Tech. Discl. Bull., Vol. 10, No. 10, Mar. 1968, pages 14891490.

Primary Examiner-Stanley D. Miller, Jr. Attorney, Agent, or Firm.loel Wall; W. C. Nealon; H. R. Berkenstock, Jr.

[5 7] ABSTRACT There is disclosed a relatively simple ramp" generator for a CRT display which generates the necessary correction current to linearize the sweep across a quasi-flat screen, As in prior art systems, an integrator develops a ramp voltage which is applied to the input of a voltage-to-current converter. However, instead of the input to the integrator being a constant current as in the prior art, the input current is constant only when the yoke current is below a predetermined positive or negative limit. Above the limit, the current at the input of the integrator is inversely proportional to the yoke current. Once the feedback becomes effective in this manner, the yoke current becomes an exponential function of time. By providing double-ended feedback, the yoke current is a symmetrical function of time with respect to its positive and negative portions, and the yoke current exhibits no discontinuities in slope.

14 Claims, 13 Drawing Figures PATENIEI] FEB I BiQFS 3.867, 3

SHEET 1 OF 3 110 11 24 l l l 39 R3 FIG.5A FIG.5B

V5 FIG.9

FIG.5C FIG.5D

SWEEP GENERATOR FOR LINEAR CATHODE RAY TUBE DISPLAY This invention relates to sweep generators for CRT displays, and more particularly to such sweep generators which are less costly than those of the prior art and yet provide linear sweeps across the face of a nonspherical screen.

It is well known that for a curved CRT screen, the deflection distance is proportional to the deflection yoke current. Consequently, the sweep current may be a straight ramp. But for a quasi-flat screen, the ramp voltage must be corrected in order to achieve a linear sweep. Prior art sweep correction circuits have often been relatively complex, have not completely solved the linearity problem, and have often been limited to particular sweep speeds.

It is a general object of my invention to provide a current sweep generator for a CRT display which is relatively inexpensive, capable of providing linearity correction independent of sweep speed, and highly flexible in the manner in which the sweep current waveform may be adjusted.

In accordance with the principles of my invention, the sweep circuit includes a conventional integrator (together with a comparator and current switch) and a voltage-to-current converter. In a conventional system of this type, the integrator-comparator-current switch sub-system provides a linear ramp output voltage which rises from a maximum negative value to a maximum positive value of the same absolute magnitude. At the end of the sweep, the output voltage is made to fall to the maximum negative value at which time another sweep begins. The linear ramp voltage is applied to the input of the voltage-to-current converter in order to derive a yoke current which varies linearly from a maximum value in one direction to a maximum value in the other.

In a conventional prior art sweep circuit, the current input to the voltage ramp generating circuit is constant; basically, it is a constant current flow at the input of the integrator which provides a linearly increasing voltage at the integrator output. In accordance with the principles of my invention, however, the current at the input of the integrator is not constant but is instead a function of the yoke current at least after the yoke current has exceeded a threshold limit in either direction. Until the threshold limit is reached, the current at the input of the integrator is constant so that the yoke current is a linear ramp. But a feedback connection from the output stage of the sweep generator to the input of the integrator causes the integrator input current to decrease as the absolute magnitude of the yoke current increases above the threshold limit in either direction. It can be shown that this feedback arrangement controls the yoke current, after the ramp limits have been exceeded, to assume the general form of an exponential which is a good approximation to the required current waveform for a linear sweep across a quasi-flat screen.

Because the feedback is ineffective until the yoke current has exceeded a threshold limit in either direction, the middle portion of the yoke current waveform is a linear ramp to provide the necessary inflection point in the overall current waveform. The overall current waveform is symmetrical about the mid-point of the sweep since the correction current required to linearize a CRT sweep is a symmetrical function. Of considerable importance is the fact that there are no slope discontinuities in the yoke current; were there to be significant slope discontinuities, a linear sweep could not be achieved. Also of considerable importance are the facts that the circuit is flexible with respect to the shape of the yoke current waveform which can be generated and the correction current component is independent of the sweep speed.

Further objects, features and advantages of my invention will become apparent upon consideration of the following detailed description in conjuntion with the drawing, in which:

FIG. 1 is a schematic representation of a curved screen and a flat screen, and will be helpful in understanding why conventional deflection circuits produce linearity errors in the case of a flat screen;

FIG. 2 depicts the shape of the yoke current waveform required for perfect linearity in the case of a curved screen, as well as the general shape of the our rent waveform required for corrected linearity in the case of a quasi-flat screen;

FIG. 3 depicts a voltage ramp generator which is a conventional prior art circuit and which is used in the illustrative embodiments of the invention;

FIG. 4 depicts a conventional prior art voltage-tocurrent converter which is utilized in the illustrative embodiments of the invention;

FIGS. 5A 5D depict the current flow in the circuit of FIG. 4 for various yoke current magnitudes;

FIG. 6 depicts an illustrative embodiment of my invention;

FIG. 7 depicts the integrator input current as a function of positive yoke current in the circuit of FIG. 6.;

FIG. 8 depicts the complete integrator input current function for both positive and negative values of yoke current;

FIG. 9 depicts a configuration for varying the operation of the circuit of FIG. 6; and

FIG. 10 is a complete circuit which is the preferred embodiment of my invention.

FIG. 1 is a schematic representation of a curved screen and a flat screen, and an examination of it reveals why conventional deflection; circuits produce linearity errors for a flat screen. In a magnetic deflection system, the beam deflection angle 0 is related to the current I in the deflection coil by the equation sin(0)=K,l. For a curved screen, the distance d is proportional to the sine of the deflection angle, which in turn is proportional to the deflection yoke current. Consequently, the deflection a is directly proportional to the deflection current and in an ideal case there are no linearity errors.

But for a flat screen, the deflection distance d is proportional to the tangent of the deflection angle. Thus, d K,tan(6) K,sin(0)/ l(sin(6)) K 1}- V lK F K (I+K.,1 It is apparent that the distance d while proportional to the deflection current, is also proportional to odd powers of the deflection current.

In order to obtain a linear displacement d, as a function of time, the deflection current must be a non-linear function of time. In the case of a cathode ray tube with a quasi-flat face, the function can be determined experimentally.

FIG. 2 depicts the shape of the yoke current waveform required for perfect linearity in the case of a curved screen as well as the general shape of the current waveform required for corrected linearity in the case of a flat or any imperfectly curved screen. Both plots in FIG. 2 depict the yoke current I as a function of time, assuming a sweep of duration T. The yoke current required in the case of a curved screen is shown by line 10. The current increases linearly from a maximum negative value to a maximum positive value, with the deflection current being zero at the midpoint of the sweep. Following the sweep, the yoke current during flyback, represented by dotted line 12, need assume no particular shape as far as linearity considerations are concerned because the electron beam is blanked during flyback. Curve 14 shows the'general shape of the yoke current waveform required for a linear deflection in the case of a non-spherical screen. At the two extremities of the sweep the yoke currents must be smaller in magnitude than the respective current values required for linear deflection in the case of a spherical screen.

A conventional sweep circuit for a cathode ray tube consists of two parts. The first is a sweep generator for developing a ramp voltage. The second is a voltage-tocurrent converter for developing the yoke current. The circuit of my invention also includes these two circuits. Accordingly the prior art circuits will be considered before the circuit of the present inventions is set forth.-

FIG. 3 depicts a voltage ramp generator, the upper half of which is an integrator, and the lower half of which is a comparator with hysteresis (also known as a Schmitt trigger) and a current switch. It is assumed that current I, is constant, that is, input terminal 8 is connected to a constant current source of magnitude It is also assumed that initially transistor O1 is nonconducting so that current 1 is 0. Since current 1 is in a direction away from the minus input of operational amplifier 10, and the operational amplifier acts as an integrator as a result of the feedback connection of capacitor C l, the voltage at the output of the operational amplifier is in the form of a ramp of positive slope.

If the power supply for operational amplifier 14 is a source of magnitude B (not shown), then this potential appears at the amplifier output when the potential at the minus input is less than the potential at the plus input. Resistors R1 and R2 comprise a voltage divider to feed back potential B at the output of the amplifier to the plus input. Thus the potential at the plus input has a magnitude (B) (Rl/(Rl+R2))=B'. When the output potential of amplifier 10 is less than B the minus input of operational amplifier 14 is at a lower potential than the plus input so that the output of amplifier 14 is positive. The positive potential at the base of transistor 01 holds this transistor off so that current 1 is zero as assumed initially.

When the potential of the ramp voltage at the output of operational amplifier 10 and hence at the minus input of amplifier 14 reaches the threshold level B applied to the plus input of operational amplifier 14, the output potential of amplifier 14 switches from +B to -B. The negative potential which now appears at the base of transistor Q1 turns this transistor on, the emitter of the transistor being held at a positive potential through the voltage divider network consisting of resistors l8 and which are connected to potential source 16. Transistor 01 functions as a constant current source to supply a non-zero current I to the junction of input terminal 8 and the minus input of amplifier 10.

Since current 1 is constant and current 1 exceeds current 1,, there is now a constant current which flows into capacitor C1. The output potential of the integrator comprising amplifier 10 and capacitor C1 now starts to decrease linearly from the maximum positive potential toward a lower level.

The potential which is fed back from the output of amplifier 14 to the plus input is (B)(Rl/(Rl+R2)). As soon as the ramp voltage at the output of amplifier 10 drops to this level, the output of the amplifier 14 switches in potential from B to +8. At this time, the ramp of positive slope is initiated and the overall cycle is complete. The ramp generator of FIG. 3 is a conventionah'oircuit, whoserates of r'iseandfall can'b' adjusted to be different for positive and negative slopes. Thus the output voltage at terminal 22 can be used for controlling a CRT sweep. The circuit consists basically of an integrator (amplifier 10 and capacitor C1), a comparator (amplifier l4, and resistors R1 and R2), and a current switch (transistor Q1).

It should be noted that ordinarily it would be sufficient to connect input terminal 8 through a resistor to a negative potential source. Since the minus input of operational amplifier 10 is a virtual ground, the current which flows out of terminal 8 would be constant. No such circuit is shown in FIG. 3, however, because for reasons which will become apparent below, a special current source is required in accordance with the principles of the invention.

The circuit of FIG. 4 is a conventional voltage-tocurrent converter. Assume initially that terminal 32 is grounded. Due to the very high gain of operational amplifier 30, the minus input is similarly grounded. With a ground potential appearing at the junction of yoke 34 and resistor R no current flows through the resistor. Similarly the yoke current 1,, is also zero and the junction of resistors R and R is held at ground potential. Constant current source 28 supplies a constant current 1,, which flows from the output of operational amplifier 30 through the two diodes D1 and D2 and resistor R,,. (In the following analysis, the base currents of transistors Q2 and Q3 are neglected.) Assuming that the voltage drop across the two diodes equals the combined voltage drops across the base-emitter junctions of transistors Q2 and Q3, it is apparent that the voltage drop across resistor R,, equals the sum of the voltage drops across the two resistors R and R Since the voltage drop across resistor R is fixed at I R at all times when both of transistors Q2 and Q3 are on the sum of the voltage drops aaoss resistors R and R must be constant. In the quiescent state, since current I,, is 0, the same current flows through resistors R and R and the magnitude of current source 28 is selected such that transistors 02 and 03 can function as a class B amplifier.

For illustrative purposes, it is assumed that when the plus input of operational amplifier 30 is grounded, 1,, 1, 10 milliamperes. This situation is illustrated schematically in FIG. 5A. The upper arrow represents the current I and the lower left arrow represents the current I both of which are 10 milliamperes. The rightmost current represents I, and in the quiescent state this current is zero.

Assume now that the voltage at terminal 32 starts to increase linearly with positive slope. More current now flows through transistor Q2 and this situation is illustrated schematically in FIG. 58 where it is assumed that the emitter current of transistor Q2 has increased from milliamperes to 11 milliamperes. Since the combined voltage drops across resistors R and R must still be constant at a value I,,R,, and resistors R and R are equal in magnitude, it is apparent that the sum of the currents which flow through the two resistors must always be 20 milliamperes just as it is in the quiescent state illustrated in FIG. 5A. Consequently, with 11 milliamperes flowing through resistor R 9 milliamperes must flow through resistor R and the yoke current must be 2 milliamperes FIG. 5C illustrates t he current flow when the input voltage at terminal 32 is sufficiently high to control a flow of 15 milliamperes through the emitter of transistor Q2. At this time 1,, is 15 milliamperes, I, is 5 milliamperes, and I0 milliamperes flow through yoke 34. Eventually, as illustrated in FIG. 5D, the current through transistor Q2 reaches milliamperes, at which time no current flows through resistor R, and transistor Q3 turns off. At this time, all of the emitter current of transistor Q2 flows through yoke 34. As the input potential at terminal 32 continues to increase, the current which flows out of transistor Q2 increases linearly, and all of the current flows through the yoke with transistor Q3 remaining off.

The above description is applicable to the case in which the input voltage at terminal 32 rises linearly with positive slope from a ground level, corresponding to the time interval T/2 through Tin FIG. 2. Similar remarks apply to the first half of the sweep waveform due to the symmetry of the circuit of FIG. 4. For low level negative input voltages, both of transistors Q2 and Q3 conduct, but Q3 conducts more heavily. Consequently, the yoke current 1,, is in the opposite direction to that shown in FIG. 4 with current being greater than current 1,,. Whenever the negative yoke current exceeds 20 milliamperes, transistor Q2 does not conduct at all, and the entire yoke current flows through transistor Q3.

Although a quiescent current of IO milliamperes has been assumed to flow through resistors R and R g, the sweep circuit of FIG. 4 functions in a comparable way no matter how low the quiescent current which flows through resistors R, and R, The lower the quiescent current, the lower the absolute magnitude of the yoke current which controls the turn-off of one of transistors Q2 or Q3. But in any case, with a ramp input at terminal 32 of positive slope which rises from a maximum negative potential to a maximum positive potential of the same absolute magnitude, the yoke current rises linearly from a maximum negative value to a maximum positive value of the same absolute magnitude. Since the output voltage at terminal 22 of the circuit of FIG. 3 is of the required ramp form, it is apparent that if terminal 22 of FIG. 3 is connected to terminal 32 of FIG. 4, and a constant current I, flows out of terminal 8 in the circuit of FIG. 3, then the yoke current will be linear and of the form illustrated by plot 10 in FIG. 2.

In the illustrative embodiment of the invention shown in FIG. 6, the circuits of FIGS. 3 and 4 are connected as described above. However, the circuit includes some additional components. The collectors of transistors Q2 and Q3, rather than being connected directly to po tential sources 24 and 26, are connected to these sources through respective resistors R3 and R4 whose magnitudes are equal. Also, instead of providing a constant current source of magnitude 1,, the minus input of operational amplifier 10 is connected to the collector of transistor Q4 as shown. Resistors R5, R6 and R7, together with diode D3, control the current flow through transistor Q4. Of primary importance is the fact that resistors R5 and R6 are returned to positive and negative potential sources 24 and 26 through respective rcsistors R3 and R4, rather than being connected directly to these potential sources.

With reference to FIGS. SA-SD, it will be recalled that with a quiescent current flow of 10 milliamperes in transistors Q2 and Q3, the sum of the currents through resistors R and R is 20 milliamperes until the yoke current reaches a magnitude of 20 milliamperes in either direction. Since current 1,, flows through resistor R3 (neglecting the base current of transistor Q2) and current 1 flows through resistor R4 (neglecting the base current of transistor Q3), and since resistors R3 and R4 are of equal magnitude, it is apparent that until the yoke current reaches a magnitude of 20 milliamperes the total voltage drop across resistor R5, diode D3 and resistor R6 is equal to .Q j2I R), where I q is the quiescent current of 10 milliaihpYFes in the selected example and each of resistors R3 and R4 has a magnitude R. As the yoke current varies from 0 to 20 milliamperes in either direction, the drop across resistor R5, diode D3 and resistor R6 remains equal to (2B 2IQR) since the sum of the currents through resistors R3 and R4 is always equal to 2IQ. This potential difference (2B-2I R) determines the value of current I, which flows through transistor Q4. Assuming that the voltage drop of diode D3 equals the baseemitter drop of transistor Q4, it follows that (2B2I RV )(R6/(R5+R6))=l R7 where V is the voltage drop across diode D. The significance of this expression is that the current 1, remains constant until the yoke current reaches a value equal to 21 because until this current is reached the sum of the voltage drops across resistors R3 and R4 is constant. While the actual potentials at the junction of resistor R3 and the collector of transistor Q2, and the junction of resistor R4 and the collector of transistor Q3, change as the yoke current changes, both potentials rise or fall together so that the difference between them is constant. It is the potential difference that is solely determinative of the magnitude of current 1,.

Consequently, until the yoke current reaches the value of 20 milliamperes in either direction, current I, is constant and the yoke current is a straight ramp. With reference to FIG. 2, it would appear that the correction current (plot 14) which is required is never the same as the ideal ramp (plot 10). However, the current waveform 14 in FIG. 2 is shown exaggerated. It must necessarily have an inflection point at time T/2 during each sweep and consequently in the immediate vicinity of the intersection of the current waveform with the time axis the actual current waveform must necessarily be in the form of a ramp. The limits of the ramp in the selected example are +20 milliamperes and 20 milliamperes, which are only small fractions of the two maximum values of the yoke current in a practical sweep circuit. The 20-milliampere limits can be adjusted by selecting a different value of quiescent current l in any specific circuit. It is at the current levels of of +2l and 2l Q that the current waveform 14 in FIG. 2 departs from a ramp form.

Considering a positive current flow in the direction shown for 1,, in FIG. 4, as the yoke current continues to increase the voltagedrop across resistor R3 continues to increase and the voltage drop across resistor R4 continues to decrease (as long as 1,, remains below 20 milliamperes), as is apparent once again from an inspection of FIGS. 5A-5D. It is because one voltage drop increases by the same amount that the other voltage drop decreases that the current 1 is constant as described above. But as soon as the yoke current exceeds 20 milliamperes, no more current flows through resistor R4. There is no voltage drop across this resistor even though the voltage drop across resistor R3 continues to increase. It is at this time that the total potential difference applied to resistor R5, diode D3 and resistor R6 is no longer constant. The current 1 is now determined by the following relationship: (2B-1,, RV )(R6/(R5+R6))=I,R7, I,, 20 milliamperes. The current 1 is therefore the following function of the yoke current 1,: 1 =(2BV )(R6/(R5+R6))( l/R- 7)I,,((RR6)/(R5+R6))( l/R7). It is thus seen that the expression for 1 is linear, consisting of a positive term minus a constant multiplied by 1,.

FIG. 7 is a plot of 1 as a function of the yoke current 1,,. Until 1,, reaches a value of 21 1 is constant as shown. Once I reaches a level of 21 (20 milliamperes in the selected example) 1 starts to decrease linearly. The two sections of the 1 plot intersect each other as shown, when 1,, equals 21 and this can be verified by substituting the value of 21 for 1,, in the ramp portion of the plot.

The above discussion assumed that 1,, increases from zero to a maximum positive value. Due to the symmetry of the circuit, the same remarks apply to negative values of yoke current. Until the negative yoke current reaches a level of 20 milliamperes (21 the potential difference between the collectors of transistors Q2 and Q3 remains constant. Thereafter, the potential difference decreases so that 1 decreases as well. FIG. 8 illustrates the symmetrical 1 current as a function of I (The entire analysis is only approximate because it assumes that 1,, is equal to the current through resistor R3 or R4, while actually some of the current flows through resistors R and R6, and diode D3.)

It is important to note that FIG. 8 does not depict any current as a function of time. It simply depicts the value of current 1, as a function of the yoke current. But the relationship between the two currents enables the waveform of 1,, as a function of time to be determined.

Referring to FIG. 6, and recalling that operational amplifier 30 has a negligible potential difference between its plus and minus inputs, it is apparent that the voltage drop across resistor R, must always be equal to the voltage at the plus input of the amplifier. Since the potential difference between the plus and minus inputs of operational amplifier 10 is also negligible and the plus input is grounded, the minus input is necessarily at a virtual ground as well. Consequently, the voltage across capacitor C must equal the voltage across resistor R,,. And since the voltage across resistor R, is directly proportional to the yoke current 1,, the voltage across capacitor C1 is necessarily proportional to the yoke current.

It therefore follows that the form of the yoke current follows the form of the voltage across capacitor C1. And the form of the voltage across capacitor C1 can be determined by analyzing the voltage waveform produced by an integrator having applied to its input a current 1,. Without reference to any specific sweep period,

it is apparent from FIG. 8 that as the yoke current increases from a value of zero, the current 1 is first constant and it then decreases linearly.

When a constant current is applied to the input of an integrator its output voltage increases in the form of a ramp. But if the input current then starts to decrease, the output voltage continues to increase (since the current I, is still positive), but the rate of rise is smaller. In fact, the rate of rise continues to decrease and the voltage is in the form of an exponential. This is the required yoke current waveform from time T/2 through time T in FIG. 2. The yoke current initially rises in the form of a ramp, and when it reaches a level of 21 the current rises in the form of an exponential. Due to the symmetry of the circuit, of course, since 1 is a symmetrical function of 1,,, the 1,, waveform during the first half of each sweep is the mirror image of the waveform during the second half of the sweep. During any single sweep, 1,, has a maximum negative value initially and the .capacitor voltage (yoke current) rises exponentially. When the yoke current reaches a value of 21 the yoke current continues to increase but it now does so in the form of a ramp. There is no abrupt change in thesweep waveform because the transition from one waveform to the other is smooth. The yoke current continues to rise in the form of a ramp until it reaches a level of 21 At this point in the sweep, the yoke current begins to rise exponentially.

There are many prior art circuits for developing exponential waveforms. But what is required for a CRT sweep are symmetrical quasi-exponential waveforms without discontinuities. That is far more difficult to achieve, but it is accomplished by the circuit of FIG. 6. Moreover, the sweep waveform is adjustable; the limits of the ramp (21 and +21 are determined by the magnitude of current source 28 and the magnitude of resistor R The limits of the two exponential waveforms can be adjusted by selecting appropriate parameter values for the circuit, an example of which will be described below in connection with FIG. 10.

It should be noted that the circuit of FIG. 6 provides feedback from the output to the input. It is the yoke current which determines the value of 1,, which in turn affects the yoke current. One of the advantages of the circuit is that the circuit is independent of time; no matter what the sweep speed, within reasonable limits, the yoke current is a smooth symmetrical exponential waveform without discontinuities in slope. The feedback is effective only after the yoke current exceeds a minimum level in either direction; until the yoke current exceeds a magnitude of 21 the feedback is ineffective because the potential difference between the collectors of transistors Q2 and Q3 remains constant so that 1, remains constant.

But there are other ways to control the change-over from a ramp to an exponential form of current wave form, and one such circuit is shown in FIG. 9. By comparing the connections to resistor R3 shown in FIGS. 6 and 9, it will be apparent that the major difference is that if the circuit of FIG. 9 is substituted for resistor R3 in FIG. 6, then resistor R5 is returned through diode D4 to resistor R3 (and the collector of transistor 02) and resistor R5 is returned to potential source 24 through resistor 39. A comparable resistor-diode network is provided at the negative potential source 26 in FIG. 6. When using such resistor-diode networks, the feedback can be inhibited until a larger value of yoke current flows. Once transistor Q3 turns off, it will be recalled that the entire yoke current flows through resistor R3. In such a case, diode D4 does not conduct until after 1,,R3 exceeds the voltage drop of the diode. It is only then that the diode turns on and the feedback is effective. Similar remarks apply to resistor R4 and a negative yoke current. By using resistor-diode networks such as that of FIG. 9, the ramp portion of the 1,, waveform can be extended. Similarly, multiple diode networks can be used, not only to extend the central ramp portion of the current waveform, but even to introduce several different ramps of differing slopes. In such a case the 1 plot might have several linear components. But in all cases, because the 1 current flows from the integrator comprising operational amplifier 11) and capacitor C there are no discontinuities in the slope of the voltage across the capacitor and therefore no discontinuities in the slope of the sweep current. The use of such diode networks allows extensive control over the sweep current waveform. But no matter what diode networks are used, symmetrical current sweeps are achieved in both halves of the sweep, without slope discontinuities. Other variations which are possible are the inclusion of Zener diodes in series with the bias resistors in the circuit of transistor 04, such as a Zener diode in series with resistor R5. Such Zener diodes would allow relative changes in the constant and ramp portions of the I plot.

. R6 and R7, diode D3 and transistor 04, or comparable The magnitude of current 1, determines the sweep speed. This is because the minimum and maximum limits of the sweep current are determined by the threshold levels applied to the plus input of operational amplifier 14, and these levels are fixed. Potentiometer R5 is a production setting potentiometer which is adjusted so that any one of three predetermined sweep speeds may be selected. Zener diode Z1 with its temperaturecompensation diode module D2 functions to increase the degree of feedback from the yoke current to the integrator input current.

When switch is in the position shown, the sweep speed is 25 millimeters/second, all of the emitter current of transistor Q4 flowing through resistor R7. When switch 40 is connected to one of resistors R8 or R9, the respective sweep speed is or 100 millimeters/- second. The larger the emitter current of transistor Q4, the faster the sweep speed.

There are various resistors included throughout the circuit of FIG. 10 for the purpose of suppressing R.F. oscillations. These resistors include resistors R10, R11, R13, R16 and R17. Potentiometer R15 provides for the attenuation of the waveform at the output of operational amplifier 10 and in effect is a gain control for assuring that the sweep just fills up the screen.

Resistor R14 limits the output current of operational amplifier 14, and capacitor C2 speeds up the switching of transistor Q1. Resistors R19, R20 and R21, together with transistor Q5, comprises a standard current source in lieu of current source 28 of FIG. 6. Capacitors C3, C4 and C5 stabilize the feedback loop from the junction of yoke 34 and resistor R,,, to the minus input of operational amplifier 30. Each of transistors 02 and O3 is a Darlington pair so that a large yoke current can flow.

The magnitudes of the various components in the cir- Quit of FIG. 10 are as follows:

potential source 24: +15 volts potential sourc e 26: -15 volts R1: kohms R10: 1 kohm R19: 15 kohm R2: 15 kohms R11: 18 kohms R20: 1.8 kohm R3: 15 ohms R12: 10 kohms R21: 1.5 kohms R4: 15 ohms R13: 18 kohms R22: 68 kohm R5: 5 kohms R14: 180 kohms R23: 39 ohms R6: 330 ohms R15: 10 kohms R 220 ohms R7: 215 kohms R16: 1 kohm R 4.7 ohms R8: 215 kohms R17: 100 ohms R,.;. 4.7 ohms R9: 71.5 kohms R18: 1 kohm R 3.3 ohms D1: 1N9l4 D2: CA3039 D3: 1N9l4 Z1: 1N5246A (16v) c1; 5 uf c2. 10 pf C3: 47 of C4: 1500 pf 7 C5: .1 uf

op. amp. l0: N5556 Q1: 2N3906 Q2: MJEI 100 op. amp. I4: 748 Q3: MJE 1090 Q4: TIS97 op. amp. 30: 748 05: T1597 Yoke 34: 42 mh, 42 ohms elements for controlling the magnitude of current I in Although the invention has been described with refaccordance with the yoke current I,,,-the yoke current erence to particular embodiments, it is to be underactually being sensed by the resistors R3 and R4 which now serve an additional feedback function.

FIG. 10 is a detailed schematic of a preferred embodiment of the invention. For the most part, the elements included in the circuit of FIG. 10 are those included in the circuit of FIG. 6, and it is only the differences between the two circuits which will now be described.

stood that these embodiments are merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

What 1 claim is:

1. A sweep current generator comprising a source of current, integrating means responsive to said source current for developing a voltage which changes smoothly and continuously from a first level to a second level as a function of said source current, driver means for generating a sweep current which is a function of the voltage developed by said integrating means, switching means for resetting the voltage developed by said integrating means to said first level responsive to said voltage reaching said second level, means for sensing the sweep current generated by said driver means to develop a voltage proportional thereto, and means for controlling said source of current to be constant until the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value and thereafter for controlling said source of current to change in a direction which causes the voltage developed by said integrating means to be exponential in form.

2. A sweep current generator in accordance with claim 1 further including means for adjusting said predetermined value.

3. A sweep current generator in accordance with claim 1 wherein said sensing means includes a pair of resistors connected to sources of potential of opposite polarities and through which the sweep current flows, and said changing means is connected between said pair of resistors and is responsive to the voltage thereacross.

4. A sweep current generator in accordance with claim 3 wherein the voltage across said pair of resistors remains constant until the absolute magnitude of said sweep current exceeds said predetermined value, and said changing means controls said source current to have a magnitude which is proportional to the voltage across said pair of resistors.

5. A sweep current generator in accordance with claim 1 wherein said sensing means includes resistance means through which said sweep current flows and a pair of terminals, the voltage across said pair of terminals remaining approximately constant until the absolute magnitude of said sweep current exceeds a predetermined value and the voltage across said pair of terminals thereafter decreasing as the absolute magnitude of the sweep current increases, and said changing means includes means for controlling the magnitude of said source current to be a linear function of the voltage across said pair of terminals.

6. A sweep current generator in accordance with claim 1 wherein the plot of said source current as a function of said sweep current consists of a plurality of joined straight lines.

7. A sweep current generator comprising a source of current, means responsive to said source current for developing a voltage which changes smoothly and continuously from a first level to a second level as a function of said source current and then back again to said first level, driver means for generating a sweep current which is a function of the voltage developed by said voltage developing means, means for sensing the sweep current generated by said driver means, and means for changing the magnitude of said source current as a function of the sweep current sensed by said sensing means, said changing means controlling said-source current to be a symmetrical linear function of said sweep current.

8. A sweep current generator in accordance with claim 7 wherein said changing means is operative to control a constant source current until the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value.

9. A sweep current generator in accordance with claim 7 wherein said changing means changes said source current in a direction which causes the voltage developed by said voltage developing means to be exponential in form when the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value.

10. A sweep current generator in accorance with claim 9 further including means for adjusting said predetermined value.

11. A sweep current generator in accordance with claim 7 wherein said sensing means includes a pair of resistors connected to sources of potential of opposite polarities and through which the sweep current flows, and said changing means is connected between said pair of resistors and is responsive to the voltage thereacross.

12. A sweep current generator in accordance with claim 11 wherein the voltage across said pair of resistors remains constant until the absolute magnitude of said sweep current exceeds a predetermined value, and

said changing means controls said source current to have a magnitude which is proportional to the voltage across said pair of resistors.

13. A sweep current generator in accordance with claim 7 wherein said sensing means includes resistance means through which said sweep current flows and a pair of terminals, the voltage across said pair of terminals remaining approximately constant until the absolute magnitude of said sweep current exceeds a predetermined value and the voltage across said pair of terminals thereafter decreasing as the absolute magnitude of the sweep current increases, and said changing means includes means for controlling the magnitude of said source current to be a linear function of the voltage across said pair of terminals.

14. A sweep current generator in accordance with claim 7 wherein the plot of said source current as a function of said sweep current consists of a plurality of joined straight lines. 

1. A sweep current generator comprising a source of current, integrating means responsive to said source current for developing a voltage which changes smoothly and continuously from a first level to a second level as a function of said source current, driver means for generating a sweep current which is a function of the voltage developed by said integrating means, switching means for resetting the voltage developed by said integrating means to said first level responsive to said voltage reaching said second level, means for sensing the sweep current generated by said driver means to develop a voltage proportional thereto, and means for controlling said source of current to be constant until the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value and thereafter for controlling said source of current to change in a direction which causes the voltage developed by said integrating means to be exponential in form.
 2. A sweep current generator in accordance with claim 1 further including means for adjusting said predetermined value.
 3. A sweep current generator in accordance with claim 1 wherein said sensing means includes a pair of resistors connected to sources of potential of opposite polarities and through which the sweep current flows, and said changing means is connected between said pair of resistors and is responsive to the voltage thereacross.
 4. A sweep current generator in accordance with claim 3 wherein the voltage across said pair of resistors remains constant until the absolute magnitude of said sweep current exceeds said predetermined value, and said changing means controls said source current to have a magnitude which is proportional to the voltage across said pair of resistors.
 5. A sweep current generator in accordance with claim 1 wherein said sensing means includes resistance means through which said sweep current flows and a pair of terminals, the voltage across said pair of terminals remaining approximately constant until the absolute magnitude of said sweep current exceeds a predetermined value and the voltage across said pair of terminals thereafter decreasing as the absolute magnitude of the sweep current increases, and said changing means includes means for controlling the magnitude of said source current to be a linear function of the voltage across said pair of terminals.
 6. A sweep current generator in accordance with claim 1 wherein the plot of said source current as a function of said sweep current consists of a plurality of joined straight lines.
 7. A sweep current generator comprising a source of current, means responsive to said source current for developing a voltage which changes smoothly and continuously from a first level to a second level as a function of said source current and then back again to said first level, driver means for generating a sweep current which is a function of the voltage developed by said voltage developing means, means for sensing the sweep current generated by said driver means, and means for changing the magnitude of said source current as a function of the sweep current sensed by said sensing means, said changing means controlling said source current to be a symmetrical linear function of said sweep current.
 8. A sweep current generator in accordance with claim 7 wherein said changing means is operative to control a constant source current until the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value.
 9. A sweep current generator in accordance with claim 7 wherein said changing means changes said source current in a direction which causes the voltage developed by said voltage developing means to be exponential in form when the absolute magnitude of the sweep current generated by said driver means exceeds a predetermined value.
 10. A sweep current generator in accorance with claim 9 further including means for adjusting said predetermined value.
 11. A sweep current generator in accordance with claim 7 wherein said sensing means includes a pair of resistors connected to sources of potential of opposite polarities and through which the sweep current flows, and said changing means is connected between said pair of resistors and is responsive to the voltage thereacross.
 12. A sweep current generator in accordance with claim 11 wherein the voltage across said pair of resistors remains constant until the absolute magnitude of said sweep current exceeds a predetermined value, and said changing means controls said source current to have a magnitude which is proportional to the voltage across said pair of resistors.
 13. A sweep current generator in accordance with claim 7 wherein said sensing means includes resistance means through which said sweep current flows and a pair of terminals, the voltage across said pair of terminals remaining approximately constant until the absolute magnitude of said sweep current exceeds a predetermined value and the voltage across said pair of terminals thereafter decreasing as the absolute magnitude of the sweep current increases, and said changing means includes means for controlling the magnitude of said source current to be a linear function of the voltage across said pair of terminals.
 14. A sweep current generator in accordance with claim 7 wherein the plot of said source current as a function of said sweep current consists of a plurality of joined straight lines. 